Subject: UPGRADE: 1200XL Clearpic--video upgrade

Reprinted from the A.C.E.C. BBS (614)-471-8559
          If I were allowed only one computer, I think that I would prefer
     a 1200XL.  Only two features draw me toward the 800XL or the 130XE,
     the Parallel Buss and  the quality of the video on my 1702 Commodore.
     The 130XE makes the 1200XL  screen look like I'm using a television -
     through the tuner! Oddly enough, the  1200XL has almost a dozen
     transistors in it's video circuits, while the XE only  has 3 or 4.
     Yet, the 130XE has a much, much better output. (same transistors, 
     same signal source.... did the Warner engineers get paid by the
     component???)  Nice for us, though. We can just duplicate the XE
     circuitry in the 1200XL -  which is exactly what I will describe in
     this article. The result is a picture  that is just as clear as the
     one from a 130XE and costs almost nothing. This is  designed for a
     separate CHROMA system like the 1702. There is no provision for 
     output to a television or a composite monitor.
          Installation will consist of removing certain components and
     either  replacing them with another part, replacing them with a wire,
     or not replacing  them at all. One component has to be added at a
     point in the circuitry where no  component now exists. All others are
     mounted in existing locations.
     R21   22K     1.5K
     L15   820uh   wire
     C115  10uf    wire
     C60   100pf   ----
     R187  1meg    2.2K
     CR20  1N4148  ----
     R22   4.3K    ----
     R23   180     47  
     R24   180     100 
     R25   130     ----
     R28   130     wire
     C62   10pf    ----
     R180  180     ----
     R181  3.3K    6.8K
     C118  100pf   wire
     R182  6.2K    ----
     R183  2.2K    ----
     R184  1K      wire
     R185  1K      180 
     C119  3.9pf   ----
     CR19  1N4148  150ohm resistor
     C116  .001uf  ----
          Add a 47ohm resistor between the collector of Q11 and +5volts.
     You will have to  cut the foil right next to the pad to isolate the
     collector.   Solder a wire from the junction of CR19 (now a 150ohm
     resistor) and R180  (removed) to the unused pin on the monitor socket.
     This is the new CHROMA  source.   Pull out Q7, Q8, Q16, Q17, and Q18.
          Check all of your changes very carefully. Measure the resistance
     between  +5volts and ground to make sure that you haven't got any
     power supply faults.  (It should read over 100ohms)   Power up in
     self-test mode and correct the color by adjusting R48.
       You are now only missing the Parallel Buss.....
                     Bob Woolley

Craig Lisowski (